When processing certain fibrous materials on air-jet spinning machines, for example polyester, it is necessary to moisten the fibrous material in the spinning nozzle or before entering the spinning nozzle. Therefore, a required amount of a wetting fluid, usually water, is supplied to the spinning nozzle. However, the consumption of the wetting liquid during spinning is very low, in the order of tenths of ml/min, and it is not continuous because the wetting liquid is supplied only during spinning and spinning-in. Given the low consumption of the wetting liquid, there is a risk of the occurrence and propagation of bacteria in the distribution system, and it is therefore necessary to provide cleaning and/or disinfection of the wetting liquid so that the wetting liquid entering the spinning nozzle will be hygienically harmless, since its residues, such as aerosol, are dispersed into the environment of the spinning factory. This is currently done by adding various disinfecting chemicals, such as the disinfectant known under the trade name “Sanosil” by Sanosil AG, Switzerland, before the wetting liquid enters the humidifying system of the machine, or, to be more specific, by supplying it to the reservoir of the wetting liquid, which is arranged before the humidifying system. The disadvantages of the chemical disinfection of the wetting liquid include the costs of disinfecting chemicals and the oxidation of the liquid in the humidifying system, causing the formation of bubbles, which subsequently cause interruption of the supply of the wetting liquid to the spinning nozzle, which can negatively affect the quality of the produced yarn.